A “sound” is vibration of an object, which propagates through a substance such as air and can be sensed by an individual using hearing organs called the ears. That is to say, the sound is originated from a vibrating object. When the sound propagates by means of the air, a portion of the air is pressed as the object vibrates and this portion of the air becomes thicker accordingly. The thickened portion of the air further presses a nearby portion of the air. Thus, the thickened portion of the air keeps on migrating and eventually reaches the ears. Then, such portions of the air having reached the ears vibrate the eardrums in the ears. Signals of such vibration are transmitted to the brain, and the sound is heard at last as the brain determines the vibration as the sound.
In this way, the sound advances as a longitudinal wave which oscillates in a direction of gas molecules in the air being pulled and pressed back and forth, or in other words, which oscillates in the same direction as a direction of advance thereof. Meanwhile, when the sound propagates, portions in which the air is thin (sparse) and portions in which the air is thick (dense) are alternately transmitted as a wave. Accordingly, this longitudinal wave is also referred to as a “sparse-dense wave (or compression wave)”.
Many speakers incorporated in general audio equipment are dynamic speaker units. Such a speaker unit at least includes: a donut-shaped magnet (a permanent magnet); a voice coil inserted into a cylindrical space that corresponds to a hole on an inner side of the magnet; and a diaphragm (a cone) formed into a conical shape and attached to the voice coil. In the above-described speaker, when a voice signal flows in the voice coil, the voice coil vibrates back and forth in accordance with a waveform of the voice signal, and the diaphragm attached to the voice coil vibrates together with the voice coil. Thus, the speaker generates a longitudinal wave having a waveform equal to that of the voice signal, and thus emits a sound.
Meanwhile, there has also been known a flat face speaker in which a flat plate-shaped diaphragm (a flat face diaphragm) is fixed in a vibratable manner to a rectangular frame. Such a flat face speaker generates a sparse-dense wave (a longitudinal wave) by pressing the air in a wide area in parallel byway of vibration of the flat face diaphragm, thereby emitting a sound (see Patent Document 1, for example).
In the meantime, the number of hearing-impaired individuals is increasing, including those with an aging-related hearing difficulty attributed to aging, an organic hearing difficulty with an impairment of any of the external ear, the middle ear, the internal ear, the cochlear nerves, and the like, a functional hearing difficulty attributed to stress, and so forth. The number of hard-of-hearing individuals in Japan is allegedly estimated to reach some 20 million. Such a hard-of-hearing individual may face not only with a risk of becoming hearing impaired but also with a difficulty in clearly distinguishing sounds which the individual manages to hear. Accordingly, the individual may fail to react to words of a companion, mislead the companion as a result of making an irrelevant response without an understanding of the contents of conversation, or spoil the conversation by frequently asking to repeat what the companion says in the middle of a talk. Such situations may hinder smooth communication. As a consequence, the hard-of-hearing individual is apt to hesitate talking with people without realizing, and thus to reduce opportunities of meeting people or to stay at home without going out. These phenomena are considered to create a problem of isolation and alienation from the society.
In general, hearing aids are used as a way to alleviate discomfort of the hearing impairment. On the other hand, in order to watch television, there is provided an FM transmitter to be connected to an earphone jack of a television, so as to transmit voices on FM radio waves and to allow an FM radio at hand to receive and acquire the voices on television.
Nevertheless, hearing aids are not preferred very much due to reasons such as being “troublesome” and “embarrassing to put them on” and are rather used reluctantly. Besides, there are quite a lot of hearing aids that pick up noise at the same time and rather end up in more stress.
On the other hand, the use of the FM transmitter is likely to require troublesome tasks of preparing the FM transmitter and installing the FM transmitter every time a user watches television. Moreover, an output destination of the sounds is switched as a result of connecting the FM transmitter to the earphone jack, which leads to a problem that a hard-of-hearing individual and a hearing individual cannot watch television together comfortably.
In the meantime, there is also a report that a sound is emitted not only as the sparse-dense wave (the longitudinal wave) but also as a transverse wave which oscillates in a direction orthogonal to the direction of advance. The transverse wave has features that attenuation of a sound in terms of a distance from a sound source is less than that of a longitudinal wave, and that it is audible to hard-of-hearing individuals. Moreover, regarding the transverse wave, there is another feature that two transverse waves or a transverse wave and a longitudinal wave do not interfere with each other.
In this regard, there has been proposed a speaker to emit a transverse wave audible to a hard-of-hearing individual, which includes a housing having a hollow structure, a drive unit housed in the housing, and a curved diaphragm formed by bending a flat plate and erected on a surface of the housing. Here, the speaker is configured to transmit vibration of the diaphragm of the drive unit to the curved diaphragm and the housing, and to emit a sound from the diaphragm of the drive unit, the curved diaphragm, and the housing (see Patent Document 2, for example).
However, the speaker described in Patent Document 2 uses the two diaphragms, namely, the diaphragm of the drive unit and the curved diaphragm erected on the face of the housing. Thus, the speaker is configured to generate a longitudinal wave audible to a hearing individual from the diaphragm of the drive unit, to generate a transverse wave audible to a hard-of-hearing individual from the curved diaphragm, and to emit sounds respectively therefrom. As a consequence, the speaker described in Patent Document 2 is not made capable of emitting a sound audible to both the hard-of-hearing individual and the hearing individual by using one diaphragm.
Meanwhile, an ideal way of radiating a large and clear sound is to transfer kinetic energy for driving the diaphragm of the driver transformed from electric energy entirely to the curved diaphragm side. However, there are various factors in the speaker of Patent Document 2 which are likely to reduce energy transfer efficiency when kinetic energy transformed from electric energy of a voice signal is transferred to the curved diaphragm side. Accordingly, the speaker of Patent Document 2 is thought to be unable to emit a larger and clearer sound because the speaker cannot efficiently transmit movement (the vibration) of the diaphragm of the drive unit to the curved diaphragm side.
Specifically, in the case of the speaker of Patent Document 2, an upright rod holds the curved diaphragm having the flat plate shape. Accordingly, the curved diaphragm is thought to be susceptible to an external force and the like, and is highly likely to deteriorate its energy transfer efficiency. Moreover, the drive unit is installed in a suspended state on an upper face of the housing. Accordingly, the drive unit is thought to be installed precariously and to be unable to sufficiently transfer the kinetic energy for driving the diaphragm of the driver to the curved diaphragm side. Furthermore, of the components constituting the drive unit, a frame is fixed to the diaphragm, and the frame is also fixed to a magnetic circuit attachment plate. Accordingly, this configuration is thought to restrict movement of the diaphragm of the drive unit generated by a magnetic circuit located on the magnetic circuit attachment plate, and to reduce (offset) the kinetic energy to be transferred to the curved diaphragm side by recoil of the diaphragm on the drive unit.